RFR: 8222145: ZGC: Introduce SoftMaxHeapSize flag

[Per Liden]

I realized that I also need to file an CSR for this, which I'll do.

/Per

On 4/9/19 11:13 AM, Per Liden wrote:
> This patch introduces a manageable SoftMaxHeapSize flag. When set, the 
> GC should strive to not grow heap size beyond the specified size. But as 
> the flag name suggests, this is a soft limit. In other words, the GC is 
> allowed to grow the heap size beyond SoftMaxHeapSize if needed.
> 
> The SoftMaxHeapSize is not be allowed to be set to a value smaller than 
> min heap size (-Xms) or greater than max heap size (-Xmx). When not set 
> on the command-line, this flag defaults to the max heap size.
> 
> There are a few different use cases where this flag could be useful. For 
> example, when you want to keep the heap footprint down, while 
> maintaining the capability to deal with a temporary increase in heap 
> space requirement. Or when you want to play it safe, with lots of 
> margin, to increase confidence that you will not run into an allocation 
> stall because of an unforeseen increase in allocation rate.
> 
> This flag also fit well with the coming JEP 351: ZGC: Uncommit Unused 
> Memory.
> 
> Since the flag is manageable, the soft max heap size can be adjusted 
> (for example via jcmd VM.set_flag) in a live JVM.
> 
> This option is not prefixed with Z, since other GCs will likely use this 
> in the future too (at least G1).
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8222145
> Webrev: http://cr.openjdk.java.net/~pliden/8222145/webrev.0
> 
> /Per